diff --git a/src/eval.c b/src/eval.c
index 3f9db7d..08603ca 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2092,14 +2092,15 @@
 
 	    if (lp->ll_di == NULL)
 	    {
-		/* Can't add "v:" variable. */
-		if (lp->ll_dict == &vimvardict)
+		// Can't add "v:" or "a:" variable.
+		if (lp->ll_dict == &vimvardict
+			 || &lp->ll_dict->dv_hashtab == get_funccal_args_ht())
 		{
 		    semsg(_(e_illvar), name);
 		    return NULL;
 		}
 
-		/* Key does not exist in dict: may need to add it. */
+		// Key does not exist in dict: may need to add it.
 		if (*p == '[' || *p == '.' || unlet)
 		{
 		    if (!quiet)
@@ -7919,14 +7920,14 @@
     }
     else		    /* add a new variable */
     {
-	/* Can't add "v:" variable. */
-	if (ht == &vimvarht)
+	// Can't add "v:" or "a:" variable.
+	if (ht == &vimvarht || ht == get_funccal_args_ht())
 	{
 	    semsg(_(e_illvar), name);
 	    return;
 	}
 
-	/* Make sure the variable name is valid. */
+	// Make sure the variable name is valid.
 	if (!valid_varname(varname))
 	    return;